Orbital order phase transition in $Pr_{1-x}Ca_xMnO_3$ probed by photovoltaics
The phase diagram of $Pr_{1-x}Ca_xMnO_3$ is modified x $\le$ 0.3, which suggests a reevaluation of the phase diagram of other manganites in that doping region. Rather than an orbital ordered phase reaching up to high temperatures of approximately 800-1100 K, we propose a loss of spontaneous orbital order already near room temperature. Above this temperature, the phase is characterized by a finite orbital polarization and octahedral tilt pattern. The tilt pattern couples to the Jahn-Teller distortion and thus induces a remaining orbital order, which persists up to high temperatures, where the tilt order is lost as well. This explains the experimental observation of orbital order up to high temperatures. The reevaluation of the orbital order transition is based on observed anomalies of various physical properties at a temperatures of 220-260 K in epitaxial thin films of $Pr_{1-x}Ca_xMnO_3$ x=0.1, i.e.in the photovoltaic effect, electric transport, magnetization, optical and ultrafast transient pump probe studies. Finite-temperature simulations based on a tight-binding model with carefully adjusted parameters from first-principles calculations exhibit an orbital order phase transition at $T_{OO} \approx$ 300 K for x=0.1. This is consistent with the experimental observation of a temperature dependent change in lattice parameter for bulk samples of the same doping at 300 K for x=0.1 and 350 K for x=0, typical for a second order phase transition. Since our reassignment of the orbital order phase transition towards lower temperatures challenges a well-established and long-accepted picture, we provide results of multiple complementary measurements as well as a detailed discussion.